Thermoplastic elastomer of a polyolefin and a monoolefin copolymer rubber which are cured with an organic peroxide crosslinking agent alone or in the presence of a coagent are known. U.S. Pat. No. 3,456,038 discloses such a composition comprising an atactic polypropylene and an ethylene-propylene copolymer rubber (EPR) cured with a peroxide and a coagent. However, the reference teaches that only atactic polypropylene is compatible with EPR and not isotactic, crystalline polypropylene because isotactic polypropylene depolymerizes when admixed at elevated temperatures with peroxide curing agents, which produce free radicals. Also, the crystalline polypropylene has a high melting point and therefore, would not be compatible with the ethylene-propylene rubber at conventional curing temperatures of about 320.degree. F.
U.S. Pat. No. 4,267,080 discloses a polyethylene composition comprising an ethylene polymer and a rubber and/or an elastomer with a crosslinking agent, such as an organic peroxide, and at least one crosslinking aid selected from the group consisting of 1,2-polybutadiene, triallyl cyanurate and triallyl isocyanurate. The use of a crystalline polypropylene, instead of polyethylene, in combination with the ethylene-propylene rubber is not taught or suggested by the reference.
In the above-mentioned references, the method of curing is static, not dynamic. However, it has been disclosed that a thermoplastic elastomer consisting of a polyolefin and a monoolefin copolymer rubber having desirable characteristics can be obtained by dynamically partially curing the two polymers. U.S. Pat. No. 3,806,558 describes the production of a thermoplastic elastomer by mixing a monoolefin copolymer rubber, typified by ethylene-propylene copolymer rubber (EPM) or ethylene-propylene-non-conjugated diene terpolymer rubber (EPDM), with a polyolefin resin, typified by polyethylene or polypropylene, and a conventional free radical generating curing agent alone or in conjunction with the usual coagents, while masticating the mixture resulting in a dynamically partially cured composition. The curing conditions are such that the composition is crosslinked to the state where it becomes almost insoluble in the usual solvents for uncured blends. Similarly, U.S. Pat. No. 4,732,940 and U.S. Pat. No. 4,785,045 disclose a dynamically partially cured composition comprising a polyolefin resin and a monoolefin copolymer rubber obtained by dynamically curing in the presence of an organic peroxide crosslinking agent and a crosslinking aid.
While the dynamically partially cured thermoplastic elastomer of the above-mentioned prior art give improved properties over thermoplastic elastomer statically cured, they still lack a good balance of overall properties, particularly tensile strength and modulus.